What is a force?
A force is a push or a pull that one object exerts on another. A force can:
- Change an object's speed (make it speed up or slow down)
- Change an object's direction of motion
- Change an object's shape (stretch, squash, bend, or twist it)
- Start or stop motion altogether
Force is measured in newtons (N), named after Sir Isaac Newton. Force is a vector quantity — it has both magnitude (size) and direction.
Contact forces
Contact forces require the objects to be touching each other:
- Friction: a force that opposes motion between two surfaces in contact. Friction always acts in the opposite direction to motion (or attempted motion).
- Applied (muscular) force: a push or pull applied directly by a person or animal, e.g. pushing a trolley or lifting a schoolbag.
- Normal force: the support force a surface exerts on an object resting on it, acting perpendicular (at a right angle) to the surface.
- Tension: the pulling force transmitted through a string, rope, or cable when it is pulled tight, e.g. a rope in a tug-of-war.
Non-contact forces
Non-contact forces act on an object without the objects needing to touch:
- Gravitational force (gravity): the force of attraction between any two objects with mass. On Earth, gravity pulls all objects towards the centre of the Earth — this is why objects fall and have weight.
- Magnetic force: the force between magnets, or between a magnet and a magnetic material, acting at a distance.
- Electrostatic force: the force between electrically charged objects — like charges repel, unlike charges attract, even without touching.
Weight is actually the force of gravity acting on an object's mass. Weight = mass × gravitational field strength (W = m × g), and is measured in newtons — not to be confused with mass, which is measured in kilograms.
Representing forces as vectors
Because force has both magnitude and direction, we represent it with an arrow:
- The length of the arrow shows the magnitude (size) of the force — a longer arrow means a bigger force.
- The direction the arrow points shows the direction the force acts in.
Force diagrams: longer arrow = bigger force · arrow direction = force direction
Balanced vs unbalanced forces
When more than one force acts on an object, we must consider their combined effect:
- Balanced forces: forces that are equal in size and opposite in direction. They cancel out, so there is no change in the object's motion — it either stays at rest or continues moving at a constant speed in a straight line.
- Unbalanced forces: forces that do not cancel out completely. An unbalanced force causes the object to accelerate — it speeds up, slows down, or changes direction.
Example: A book resting on a table has its weight (down) balanced by the normal force from the table (up) — the book stays still. If you push a trolley and the applied force is bigger than friction, the forces are unbalanced and the trolley speeds up.
Resultant force
The resultant force is the single overall force that would have the same effect as all the individual forces acting on an object combined. For forces acting along the same straight line:
- Forces acting in the same direction are added together.
- Forces acting in opposite directions are subtracted from each other, and the resultant acts in the direction of the bigger force.
Resultant force = sum of forces in one direction − sum of forces in the opposite direction
Example: Two learners push a box. One pushes right with 40 N, the other pushes left (opposing) with 25 N. Resultant force = 40 − 25 = 15 N to the right → the box accelerates to the right.
If the resultant force is zero, the forces are balanced and there is no change in motion. If the resultant force is not zero, the object accelerates in the direction of the resultant force.